A step toward tomography of protons
DOI: 10.1063/1.4796379
In medical tomography, a planar slice of tissue is imaged and its three-dimensional structure is built up by stacking the planar views. By analogy, physicists at the Thomas Jefferson National Accelerator Facility in Virginia are attempting to image the quarks inside protons, one planar slice at a time, in momentum space. The probe is an intense beam of electrons and the target is liquid hydrogen. The physicists seek rare events called deeply virtual Compton scattering (DVCS) in which an incoming electron sends a virtual photon (a high-energy gamma ray) ahead of it. The virtual photon scatters from one of the elementary quarks in the proton and a real gamma ray re-emerges, leaving the target proton intact. Detection of the outgoing electron and photon provides information about the status of quarks inside the proton. For example, the probability of a quark’s spatial position inside the proton, transverse to the direction of the virtual photon, can be related to the angle and energy of the outgoing gamma ray. In high-energy electron scattering, the square of the transferred four-momentum (Q 2) from electron to quark determines the spatial resolution. Beyond a certain point, however, a larger Q 2 does not provide greater resolving power because individual point-like quarks have no structure of their own. That the present experiment shows the scattering to be independent of Q 2 above about 2 GeV2 is evidence that the technique is indeed imaging the distribution of the elementary quarks inside the proton. (
